Photochromic glasses were invented some thirty years ago, as described in U.S. Pat. No. 3,208,860 (Armistead et al). Because the main application for photochromic glasses is the fabrication of ophthalmic lenses, extensive research and development work was performed in order to design glass compositions which, not only show fast darkening and fading, but also have a refractive index of 1.523, the industrial standard for common lenses.
However, during the past few years, extensive research was devoted to the development of photochromic glasses of higher refractive index. The very significant benefit resulting from the use of higher refractive index glasses bears on the possibility to reduce the lens thickness, while keeping the same lens corrective power. Several types of compositions were developed, as summarized by the following works:
U.S. Pat. No. 3,703,388 (Araujo) describes the use lanthanum borate compositions containing silver halide; and
U.S. Pat. No. 3,999,996 (Faulstich et al) concerns the use of high lead oxide concentrations in aluminoborosilicate base compositions containing silver halide.
For various reasons, the glasses that came out of the first research efforts did not fulfil all commercial expectations. More recent investigations were focused on the use of two or several oxides of the group TiO.sub.2, Nb.sub.2 O.sub.5 and ZrO.sub.2, with additions of alkaline earths, in order to raise the refractive index. The addition of TiO.sub.2 yields a rapid increase of the refractive index without increasing the glass density as much as BaO, Nb.sub.2 O.sub.5 and ZrO.sub.2 do. The resulting lens is lighter in weight. The most widely sold photochromic ophthalmic lenses having a refractive index of about 1.6 contain over 2% TiO.sub.2 in their composition, and are brown in the darkened state. Unfortunately, the presence of TiO.sub.2 in these glasses confers a yellowish tint to the glass, which should be essentially colorless in the virgin/faded state. In other words, present commercial photochromic glasses exhibiting refractive indices close to 1.6 and darkening to a brown colour when they are exposed to actinic radiation are not as colorless in their virgin and faded states as commercial photochromic glasses exhibiting a refractive index of 1.523 which darken to a brown colour when they are exposed to actinic radiation.
Consequently, one of the objectives of the present invention was to provide transparent photochromic glasses with refractive indices close to 1.6, which darken to a desired colour when exposed to an actinic radiation, and which fade to an essentially colorless state (at least equivalent to that presented by photochromic glasses exhibiting a refractive index of 1.523}, while demonstrating equivalent or improved photochromic performances.
Another objective was to provide glasses of the above described type which display a brown colour in their darkened state and exhibit a virgin/faded coloration less than that of 1.523 index glasses.